The first code creates an id that directs how the image will behave. It is anchored to the top-left, which is exactly how it is currently on your site.
The min-width and min-height = 100% preserves the aspect ratio. You could also play with the max-width: 100% setting, because I'm not sure what it is you're trying to do with the rest of your layout.

Remember, this code resides in your css file, not in your html file.

The second code protects what's already there. If you screw up, just delete what you've written and unquote what was there earlier. Don't make mistakes here!

Don't forget to save the file.

The third code places the image right at the beginning of your html document, and it will behave as instructed by the first code. Try it, and see how it behaves.

Play with min-width, min-height, max-width and max-height, depending on how you want the rest of your site to behave. E.g., if you specify the max-width: 100%, the image will always display as 100%, but then you're not restricting its minimum size.

Some other observations:

The way your site is designed, I don't advise a left anchor for the background image. You should ideally place the background image in the middle of the browser, and the main content will always be centered. Right now, there's a lot of white space on my 1920x1080 monitor, and the content isn't centered.

Unfortunately, my knowledge of blogger is limited, so I can't direct you on how and where to find the right files.

So, here's a workaround:

Make a long and thin image of the green+beige strip, and let it repeat horizontally. Add the heads, primates, and humans (are they filmmakers?) as separate png images anchored according to the main body. This way, you never have to worry about the placement of the images. Simple.

Hey Sareesh, many thanks for your support. I´ve been doing some tests and I´ve managed to fix a lot of of what I wanted. At the moment all I want is to center each post: Papos de Mentes. If you scroll down you will see that the video is slightly to the left. There is a native option to descrease the box size but not to center it.